Help! > Specific Engine Help

Built up crankshaft feedback

(1/9) > >>

petertha:
Soliciting feedback from you talented experts. I drew this crankshaft up as a possible built-up equivalent version of what was originally part machined from solid bar stock. It's for Jung, horizontally opposed twin, 2x30=60cc total. It just occurred to me what a hefty chunk of material this would be to accommodate the counterweight OD. It measures 48mm dia x 188mm end to end for scale. So a lot of swarf & machining operations & after that, the usual possibility of post-machining distortion even if made from something like Stressproof steel. There are some additional machining subtleties specified which I have omitted for now.

Anyway, I am contemplating making a built up version & here is where I'm at idea wise
- the 3 vertical web elements would start out as basically identical circular discs
- the webs would each have the same center shaft hole and 2 crankpin holes on either side
- visualize that the center shaft would be one single full part bonded to the 3 web elements
- visualize that the crank pins would span 3 elements & be bonded to all 3

Then the tricky bit LOL
- saw cut the segments of crankpins & main shaft elements that don't belong
- with the alignment magically correct & no spring back & assembly sufficiently rigid, drill & insert permanent locking dowel pins or similar to make it mechanically stable for running
- finish turn the sawn ends flush to the web faces leaving the bonded slugs permanently intact in the webs

The purpose of the hole arrangement in the webs is to serve double duty - either hold the crankpin where it belongs or act as an alignment aid to the overall assembly as much as possible. I recognize the shaft / hole fits could be tricky. They cant be anything like press fit or it wont slide through the web elements easily. Too loose & the retainer glue would shear. So I'm thinking 'just enough' say 0.001" oversize hole to provide me some leeway but easily within the limits of what typical Loctite retainer allows. The objective of retainer is just to freeze the alignment position sufficiently to the point mechanically pin the parts & not have anything come loose during this operation.

Now that's the next big question - how to pin it. I watched a video on a Bugatti CS & they had these dowel pins with a flat on one side that wedged against a mating surface near the crankpin. I don't think that would scale easily to my size. I don't ever need this CS to come apart & I don't have the requirement to accommodate a middle bearing (although I've been contemplating that too if it could fit). So could I get away with dowel pins on all the mating surfaces? How should they be positioned for maximum strength?

petertha:
If I did not explain it quite clearly, hopefully this sketch helps. The crank pin elements would be oversize & slide through all webs & same goes for center crankshaft rod element. Bonded in place, pinned etc.
Then X shows the bits to be removed & finished flush.

crueby:
The biggest change I would suggest is to the order of your steps. Wait till after everything is pinned (and/or silver soldered, loctited, whatever) to cut away the un-needed sections of the main shaft. That will keep everything aligned during all the other work. The ends of the cut shaft will need some finish milling or filing to get them flush with the inner faces of the webs so they don't catch on the connecting rod end.


I've built both single-piece and built-up crankshafts, and prefer the built-up ones, especially if the crank throw is large. Lots less in the way of tricky setups on the lathe. Personally I prefer taper pins on the connections (with some high strength loctite for extra confidence). The taper pins do require special drills or reamers, so roll pins are another option. Some people like to silver solder the connections, though the heat involved could cause issues on some metals. All a matter of preference. I just build steam engines, so the cranks are taking less force and speed than a gas/diesel engine would impart.

Thats my 2 cents worth!

Just saw your second post (we posted at the same time), and sawing out the un-needed parts last would leave the last little section in place in the center web - not an issue but thought I would mention it. On many engines the two sets of webs have a short length of the main shaft between them, thats what I am used to making.
 :cheers:

petertha:
Thanks. Just to clarify, the holes with red + serve as temporary, supplemental alignment holes. But also where I would just leave the slug of original shaft material in place with retainer. Its not going to be as strong as virgin stock, but hopefully better than an open hole. The green lines indicate required clearance for connecting rods.

I did think about silver soldering & maybe leaving all the alignment stock in place would be enough to mitigate heat distortion. But in my mind I was hoping to depend solely on mechanical dowelling for mechanical strength even if the retainer started to fade in the future.

So should a tapered pin go right through the web & crank pin? Would I size it to be slightly longer & then finished flush on both sides for maximum engagement strength? Aside from 'keeping things put' I'm having difficulty visualizing how to best orientate pins from a stress standpoint. Just visually eyeballing some of these 3D stress/strain visualizations, seems like the middle web segment is the rubber duck. Maybe because the bearings are closely positioned on either side of the fore-aft webs providing support & the middle accordion is seeing the con rod force. So maybe a big diameter alignment hole in the middle of center web is not a good idea. The crankshaft shaft on the front & rear webs would see mainly shear force which I suppose means bigger pin section area is better? Or maybe intercepting the crankpin through its tangent OD is better than cross drilling through the middle?

On that note, this is a cool Bugatti crankshaft video showing partial disassembly & some of the other attributes that made this engine cool. Some of this subject matter was discussed on some other forum builds, but they are even more challenging with middle bearings.

http://www.youtube.com/watch?v=RpohTIT7KSA

petertha:
And then there's the sound. OMG. I'll just drop this here LOL

http://www.youtube.com/watch?v=s0JlQeCeExs

Navigation

[0] Message Index

[#] Next page